We have produced an action spectrum of photodamage using femtosecond illumination from 700 to 900 nm by monitoring the onset of DNA synthesis inhibition in cultured HeLa cells without added dyes. Doses (Int) are defined as the nth power of the peak irradiance at the sample multiplied by the total dwell time of the focal volume in a cell. (The irradiance, I, is in photons/s((m2, and n is the number of photons required for a single excitation event, or the excitation order.) Since we have not yet determined the excitation order of damage at all wavelengths, this action spectrum is expressed with n = 2, defining two-photon events. The measured damage thresholds are orders of magnitude above typical imaging doses.